Hello,
searching through the documentation I have seen that it is possbile to update packages with vulnerabilities, but instead wanted to know if you can upgrade the packages just in case we bug, I'll explain:
For example i have mysql v5.0.0.0 installed which has a bug, there is the 5.0.0.1 version that does not have this bug, however, there is also the 5.2.0.0 version that has the bug but it also has new features, I I would like the system automatically instal me the only 5.0.0.1 solving the bug.

Is mysql on your system really that far out of date? versions 5.0.0 -5..0.2 are no longer in portage; latest stable is dev-db/mysql-s-1.7.0; testing is at dev-db/mysql-5.5.32; http://packages.gentoo.org/package/dev-db/mysql
put buggy version that is newer than the good version in /etc/portage/package.mask; for example as =dev-db/mysql-5.0.2_________________Defund the FCC.

Is mysql on your system really that far out of date? versions 5.0.0 -5..0.2 are no longer in portage; latest stable is dev-db/mysql-s-1.7.0; testing is at dev-db/mysql-5.5.32; http://packages.gentoo.org/package/dev-db/mysql
put buggy version that is newer than the good version in /etc/portage/package.mask; for example as =dev-db/mysql-5.0.2

Not, isn't an example, it possible with the option of emerge upgrade all system , but only upgrade program version with a bug, not with news fecture.
Is for do a ambient server, whre is very important a stability.